Relays are commonly used to selectively control power from a power supply to an electrical load. Generally, a relay is an electrically operated switch having at least one pair of contacts and a mechanism for opening or closing the pair of contacts based on a control signal. The pair of contacts can be disposed in a circuit between the power supply and the load such that when the contacts are closed, the circuit is complete and power from the power supply can energize the load. When the relay contacts are open, the circuit is also open preventing power from reaching the load.
Opening or closing the relay contacts while the power supply is active can cause a spike in current. Specifically, closing the relay contacts can cause high inrush currents and opening the relay contacts can cause high backrush currents or kickback power. These inrush and backrush currents can be several orders of magnitude greater than the load's steady state current level and can damage the relay contacts, the load, or any other components in the circuit. For example, high inrush currents can cause the contacts to become pitted due to arcing between the contacts. High inrush currents can also cause the relay contacts to become welded together. High backrush currents can erode the relay contacts.
One conventional solution to reducing the level of inrush and backrush currents is to open and close the relay contacts at a zero cross of the supply power voltage waveform. This method can work well with certain load types, but not with all load types. For example, closing the relay contacts at the zero cross for an inductive load can cause large current spikes as residual magnetism in the inductive load may be in phase with the supply power. The optimal switching points for activating and deactivating a relay such that inrush and backrush currents can vary from one load to another.